PhD Defense by Niels Fabrin Nymand

Title: Radar investigation of the Northeast Greenland Ice Stream - Deriving the crystal orientation fabric

Abstract:

The Northeast Greenland Ice Stream (NEGIS) is the largest ice stream of the Greenland Ice Sheet, spanning over 600 km and with a drainage basin that covers 16% of the Ice Sheet. Dynamic mass loss currently accounts for around half of the total mass loss of the Greenland Ice Sheet and is primarily driven by ice streams that transport ice from the interior to the coast. Unlike most ice streams, NEGIS is not controlled by the basal topography, making it extremely difficult to model accurately. The crystal orientation fabric (COF), describing the way ice crystals are oriented within the ice, is known to be important for understanding the flow in ice streams, as the stiffness of ice can vary by two orders of magnitude depending on COF and the orientation of applied stress. The COF evolves differently under different strains, making it valuable to get information about the flow history of a region. This thesis explores the use of polarized radar to study the COFs of NEGIS. In the summer of 2022, we conducted a comprehensive survey of NEGIS using a multi-polarized radar system that was meant to give unique insights into the dynamics of the ice stream. Two studies are presented based on this data.

The first study is a method to derive the horizontal anisotropy of the COF from previously unobserved double reflections, revealing an asymmetry in NEGIS that indicate a more variable ice stream than previously assumed. The second study is an inverse problem approach that further allows us to estimate COF orientation and anisotropy as a function of depth, with results aligning well with EastGRIP ice core data and indicating a transition to weak horizontal anisotropy near the base of the ice.

Evaluation Committee: Klaus Mosegaard (chair),  Kenichi Matsuoka, Carlos Martin Garcia

Supervisors: Dorthe Dahl-Jensen, David Lilien